Clean-up nolock.h: it doesn't serve any purpose anymore. Appropriate code moved
to x86-gcc.h and my_atomic.h.

If gcc sync bultins were detected, we want to make use of them independently of
__GNUC__ definition. E.g. XLC simulates those, but doesn't define __GNUC__.

HS/Spider: According to AIX manual alloca() returns char*, which cannot be
casted to any type with static_cast. Use explicit cast instead.

MDL: Removed namemangling pragma, which didn't let MariaDB build with XLC.

WSREP: _int64 seem to be conflicting name with XLC, replaced with _integer64.

CONNECT: RTLD_NOLOAD is GNU extention. Removed rather meaningless check if
library is loaded. Multiple dlopen()'s of the same library are permitted,
and it never gets closed anyway. Except for error, which was a bug: it may
close library, which can still be referenced by other subsystems.

InnoDB: __ppc_get_timebase() is GNU extention. Only use it when __GLIBC__ is
defined.

Based on contribution by flynn1973.

made in 227510e0.
We now use lex->last_field->interval_list directly instead.

        Item_bool fixed to behave smarter with NOT operation.

MDEV-11039 - Add new scheduling algorithm for reducing tail latencies (for 10.2)

        strings/json_lib.c added as a JSON library.
        SQL frunction added with sql/item_jsonfunc.h/cc

Rebase

Sync upstream and rebase.

On PPC64 high-loaded server may crash due to assertion failure in InnoDB
rwlocks code.

This happened because load order between "recursive" and "writer_thread"
wasn't properly enforced.

Clean-up periodic mutex/rwlock waiters wake up. This was a hack needed to
workaround broken mutexes/rwlocks implementation. We must have sane
implementations now and don't need these anymore: release thread is
guaranteed to wake up waiters.

Removed redundant ifdef that has equivalent code in both branches.

Removed os0atomic.h and os0atomic.ic: not used anymore.

Clean-up unused cmake checks.